My mum decided a long time ago she wanted to be referred to as Nana when DS was born, because her mum was always a Nana to us when she was alive. She sees DS most days and lives round the corner so he spends a lot of time with her. DS is her first grand child.
DH's mum has never mentioned what she wants to be called. We only see her every few months as she lives further away and DS is unlikely to spend much time with her. We don't do family gatherings with her and DH is not at all close to his family. MIL has sent a Christmas card to DS and signed it off Nana. AIBU to ask her to be called something else as my mum is already Nana? I think I probably am but I don't want DS to get confused at having two grandparents referred to as the same thing.
To make it harder DS is MIL's 4th grandchild and so I assume the others must all call her Nana which is why she is using it.
I'm sure I'm probably overthinking it so feel free to tell me if I am!